This invention relates to lithographic systems and techniques that are used in, for example, integrated circuit manufacturing; and more particularly, in one aspect, to measure, inspect, characterize and/or evaluate optical lithographic equipment, methods, and/or sub-systems related thereto (for example, the optical sub-systems and control systems of the equipment as well as photomasks used therewith).
In the fabrication of integrated circuit, lithography is employed to “print” circuit patterns on a wafer (e.g., silicon or GaAs semiconductor substrate). Currently, optical lithography is the predominant form of lithography used in volume integrated circuit manufacturing. Optical lithography typically employs visible or ultraviolet light to expose a given pattern (generally defined by the photomask) on the resist that is disposed on a wafer to be transferred into the substrate through resist development and subsequent process steps, for example, etching, deposition and implantation
In optical lithography, the photomask (or mask), is first written using electron-beam or laser-beam direct-write tools. The mask contains certain patterns and features that are used to create desired circuit patterns on a wafer. The process of fabricating a complete integrated circuit typically requires the use of many masks.
In the field of integrated circuit manufacturing, a common lithographic tool used in projecting an image or pattern formed in a photomask onto a wafer is known as a “stepper” or “scanner”. With reference to FIG. 1, lithographic equipment 10 (for example, a stepper) may include mirror 12, light source 14 to generate light 16 at, for example, an exposure wavelength λo. The lithographic equipment 10 may also include illumination optics 18, projection optics 20, and a chuck 22 upon which a wafer 24 is temporally secured, typically by way of electrostatic or vacuum forces, in a wafer plane. The mask 26 is positioned and optically aligned to project an image of the circuit pattern to be duplicated onto wafer 24. The lithographic equipment 10 may employ a variety of well known stepping, scanning or imaging techniques to produce or replicate the mask pattern on wafer 24.
In general, there are three stages at which the integrity of the lithography process is measured, characterized or inspected. First, the mask is inspected to determine whether the pattern on the mask accurately represents the desired mask design. Second, the optics of the stepper (for example, light source 14, illumination optics 18, and projection optics 20) are measured or characterized to confirm that they are within acceptable guidelines. Third, the pattern “printed” or formed on the wafer or die (discrete pieces of the wafer) is inspected and analyzed to determine or measure the quality of the fabrication process.
The photomasks are typically inspected first by the photomask fabricator before providing them to an integrated circuit manufacturer, and then periodically by the integrated circuit manufacturer, for example, during initial mask qualification and re-qualification. The fabricator and manufacturer tend to use standalone equipment, for example, tools made by KLA-Tencor (e.g., TeraStar series equipment) or Applied Materials (e.g., ARIS-100I equipment). This standalone equipment, among other things, assesses the accuracy or integrity of the photomask as well as its ability to produce an accurate representation of the circuit design onto the wafer or die, when used in conjunction with appropriate stepper optics and settings. While such inspection equipment may provide an accurate representation of the photomask, it tends to be quite expensive and hence its use tends to be minimized.
Moreover, such inspection equipment often employs optical imaging systems (or sub-systems) that are fundamentally different from that used by the stepper in “printing” the image on the wafer during mass production. For example, such standalone tools may include optical imaging systems that employ wavelengths that are different from optical imaging systems used in the mass production steppers. The response and/or characteristics of photomask may depend upon the wavelength of the light used to measure or detect the mask (via, for example, an aerial image). Indeed, a photomask may exhibit defects in the production stepper environment that may not be detectable in the standalone inspection tool because, for example, detection of certain contaminants depends on wavelength. That is, certain contaminants may present serious issues at the wavelength used during production but may be undetectable at the wavelength used during inspection.
The optics of the stepper are typically characterized by the manufacturer after the stepper is manufactured using grating and wavefront interference methods. The manufacturer may also employ scanning electron microscopy (SEM) techniques to measure the patterns printed, formed or projected on test wafers. In this regard, the manufacturer typically uses photomasks having specifically designed test patterns. In this way, a resist pattern developed on a test wafer may be measured using SEM techniques and compared to a known, predetermined, fixed or expected pattern.
Due primarily to complexity of the inspection techniques, the inspection procedure of the stepper tends to require or consume an extended period of time, often days to complete, and thus represents an expensive procedure for the integrated circuit manufacturer to carry out.
The integrated circuit manufacturer, however, may inspect and evaluate a stepper indirectly, using SEM inspection and analysis of the developed resist image. Here again, due to the extended test time, inspection of the stepper is not performed very often, and, as a result, there are few samples and/or data to form a reliable measure of the stepper.
Conventional techniques to evaluate the final printed circuit pattern on the wafer or die tend to require examining the pattern formed on the wafer using SEM techniques. In this regard, the characterization or verification of the accuracy and quality of the circuit pattern permits an indirect method of characterizing or verifying the mask and stepper (including optics), as well as the interactions between the mask and stepper. Because the final printed circuit pattern on the wafer or die is formed after the resist development and may be after substrate treatment (for example, material etching or deposition), it may be difficult to attribute, discriminate or isolate errors in the final printed circuit pattern to problems associated with the photomask, the stepper, or the resist deposition and/or the developing processes. Moreover, as with inspection of the optics of the stepper, inspecting the final printed circuit pattern on the wafer or die using an SEM tends to offer a limited number of samples upon which to detect, determine, and resolve any processing issues. This process may be labor intensive and presents an extensive inspection and analysis time.
Thus, there is a need for a system and technique to overcome the shortcomings of one, some or all of the conventional systems and techniques. In this regard, there is a need for an improved system and technique to inspect and characterize optical lithographic equipment, including the optical sub-systems, control systems and photomasks, that are used in, for example, integrated circuit manufacturing.
In addition, there is a need for a system and technique of photomask inspection and characterization of in-situ or in a mass production/fabrication environment. In this regard, there is a need for a system and technique to measure, sense, inspect, detect, capture and/or evaluate the aerial image of a photomask in situ—that is, in the mass production environment using the lithographic production equipment of that environment. In this way, the errors may be isolated and attributed to a given aspect of the process or system. Indeed, the causes of errors in a final printed circuit pattern may be isolated, characterized and/or measured (in, for example, the photomask, stepper, and/or resist developing process) so that appropriate corrective measures may be determined efficiently, rapidly and in a cost-effective manner. Thus, there is a need for a system and technique that permits errors in the lithographic fabrication process to be attributed or isolated to certain methods or equipment (for example, the photomask or optical sub-system) in order to facilitate appropriate and/or efficient correction of such errors in the final printed circuit pattern and thereby enhance or improve the quality, yield and cost of integrated circuits.
Further, there is a need for an improved lithographic image evaluation technique and system that overcomes one, some or all of the conventional systems and techniques. In this regard, there is a need for a system and technique to more thoroughly, quickly and/or more often evaluate and calibrate lithographic imaging systems, for example, steppers, in an efficient and cost-effective manner. In this way, the quality, yield and cost of integrated circuits may be improved.